Author: Richard Weil, MEd,
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
It seems as though the concept of a heart rate training zone has been around forever. But I wonder how many people really understand how it works. In this article, I discuss the concept and how to determine your own zone.
The Main Idea
A heart rate training zone is a range that defines the upper and lower limits of training intensities. It is calculated using an age-related predicted maximum heart rate (HRmax) and a special equation called heart rate reserve (see "Calculating a Target Heart Rate Zone" below). The values are expressed as a percentage of maximum heart rate (for example, 70% of HRmax), and the range is based on (1) metabolic systems in your body that fuel your muscles during exercise, and (2) how hard you want to train. Training from 40% to 85% of HRmax is aerobic exercise ("cardio"). Aerobic means "with oxygen." Training above 85% of HRmax is anaerobic exercise. Anaerobic means "without oxygen."
The primary fuel during aerobic and anaerobic training is fat and carbohydrate, respectively, but it is very important to understand that both fuels are burned simultaneously at virtually all levels of exercise; it is not just one fuel or the other, except at the very highest intensities (close to 100% of HRmax). Resistance exercise and sprinting are examples of anaerobic training, whereas walking and jogging are typically considered aerobic, although you could walk or jog fast enough to make it anaerobic. It's likely that you are working anaerobically (above 85%) if you're out of breath during a workout and working aerobically (less than 85%) if you're only slightly out of breath.
Maximum Heart Rate
HRmax is calculated by subtracting age from 220. The equation "220-age" yields an estimate only, since there is variability in maximum heart rates (see "Errors in Predicting Maximum Heart Rate" below). HRmax is biologically determined and declines as you age, and the correlation to age is strong; that is, if a large number of 20-75-year-old individuals walked on a treadmill to exhaustion to reach their HRmax, the distribution of heart rates would range from approximately 200 bpm (beats per minute) for the 20-year-olds down to 145 bpm for the 75-year-olds.
What Range Should I Train At?
Most people train within an aerobic exercise training zone (40% to 85% of HRmax). Aerobic capacity (endurance) will improve faster if you train closer to 85% than if you train at 65%, but some individuals don't have the capacity to start training at 85%, or they simply prefer to start training at lower values and gradually increase the intensity over the time. Some individuals may even need to start at levels as low as 40% or 50%, depending on their age, level of fitness, or body weight. But the level that you start at isn't all that relevant. What matters most is that you get started, and then over time, as your endurance improves, you can gradually increase the intensity.
The body accommodates to both low- and high-intensity workouts by increasing the activity of respiratory enzymes and other biochemical reactions in the muscles. Anaerobic training-like intervals and speed work are helpful if you want to improve your time or perform optimally in an event like a 10K run or a 50-mile bike ride because the training prepares your body for the specific anaerobic demands of the event (like when you have to sprint or climb a hill). This type of training, called "specificity of training," is effective because it mimics the type of exertion experienced during the event.
On the other hand, if health and general levels of fitness are the goal, and not performance in a road race, then there's no need to train anaerobically unless you like to push. Instead, substantial gains in health and fitness can be accrued by aerobic training between 40% and 85% of HRmax. Volumes of research prove this.
A traditional method of aerobic training is to start at the low end of the aerobic training range, say 50% or 60%, and as training continues and the heart and muscles adapt to the challenge, the intensity is progressively increased. For example, a sedentary individual might start at 60% of HRmax and remain at that level for four weeks, and then during the fifth week increase the intensity to 65% (increases of 10% of intensity and/or duration is the standard recommendation). Again, the body accommodates to the work over time, and when higher levels of fitness are desired, the intensity needs to be increased. Training heart rate zones offer a quantifiable method of guiding workouts and determining exercise intensity.
Fat Burning vs. Cardio Mode?
Perhaps no other training "technique" is more gimmicky and misleading than the "fat burning" and "cardio" modes on the control panels of exercise equipment. They are based on the biology that at lower levels of exertion a higher percentage of fat is burned compared to carbohydrate. That's because:
1. Fat is denser fuel than carbohydrate (9 calories per gram vs. 4 calories).
2. It takes more oxygen to burn fat than carbohydrate because fat is denser.
3. At lower levels of exertion, you presumably breathe in and deliver more oxygen to the muscles to burn fat.
All of the above may be true given the right circumstances, but there are problems with it when it comes to real-world exercise scenarios. First off, lots of fat is burned at all intensities within the aerobic training zone. Secondly, the terminology "fat burning" and "cardio" can confuse individuals into thinking that fat is burned only during exercise in "fat burning" mode and that no fat is burned in "cardio" mode. The fact is that you burn fat during both modes. But the major problem is that the fat-burning mode is typically too slow a workout for many people to maximize benefits. In fact, at the end of a fat-burning workout, you could end up burning fewer calories and less total fat than during a cardio-mode workout. Here's an example of what I mean.
Suppose a 150-pound moderately fit man walks on the treadmill for 60 minutes at 3.0 mph ("fat burning" mode). That's 300 calories for a 150-pound man (a 150-pound man burns 100 calories per mile whether he walks or runs). Since this man is moderately fit, he will burn approximately 60% of the calories from fat (180 calories) and 40% from carbohydrate (120 calories).
Now let's say the same 150-pound man walks on the treadmill for 60 minutes at 4.0 mph ("cardio" mode). That's 400 calories burned, with approximately 50% of the calories from fat (200 calories) and 50% from carbohydrate (200 calories). The percent of fat burned may be less at 4.0 mph than 3.0 mph because the exertion is higher and so theoretically less oxygen is delivered to the muscles.
If you examine the example carefully, you will notice that at the slower fat-burning mode the man does indeed burn a higher percentage of fat compared to cardio mode (60% v. 50%), but in cardio mode, he burns more total calories (400 v. 300) and more total fat (200 calories v. 180 calories). My suggestion is to ignore the fat-burning mode (unless you want a less intense workout). You're not going to burn more fat in this mode than in cardio mode, and it could end up being an inefficient use of your time. I suggest training as hard as you comfortably can without risking injury so that you maximize the calorie and fat burn and the overall cardiorespiratory training effect.
Calculating a Target Heart Rate Zone
I recommend the heart rate reserve method (HRR) for calculating a heart rate zone. Heart rate reserve uses the range from your resting heart rate to predicted maximum. Below is the formula and an example of the method for someone 29 years old, assuming a resting heart rate of 68 bpm and a training range of 70%. You can get other ranges if you plug in other values.
1. 220-Age = HRmax
2. Subtract resting heart rate from HRmax = Heart Rate Reserve (HRR)
3. Multiply HRR times the percent that you want to train at
4. Add back resting heart rate
Assuming a resting heart rate of 68 bpm, 29 years old, and a 70% training range:
1. 220 - 29 = 191
2. 191 - 68 = 123
3. 123 x .70(%) = 86
4. 86 + 68 = 154 bpm
In this example 154 beats per minute is 70% of HRmax. The American College of Sports Medicine recommends a training zone of 40/50%-85% of HRR for developing and maintaining cardiorespiratory fitness.
Errors in Predicting Maximum Heart Rate
Calculating target ranges in individuals over age 40 can be inaccurate because of errors in estimating HRmax due to considerable heart rate variability in older adults. This means that the popular equation to estimate HRmax, "220-age", may not be accurate in individuals older than 40 years. The error is probably due to the origin of the equation which was derived from volunteers who were most likely not representative of the general population.
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In an important study published in The Journal of the American College of Cardiology in 2001, researchers examined data from 351 studies (18,712 subjects) and determined that the "220-age" equation underestimates maximum heart rate in older adults (the older the individual the more the error). In their conclusion, the researchers suggest the following equation to estimate HRmax in older adults:
208 - (0.7 x AGE)
Using this new equation, a 70-year-old's predicted HRmax is 159 bpm (208 minus 49). In the original "220-age" equation, it's 150 bpm (220 minus 70), or nine beats per minute lower. The problem with using a lower HRmax to calculate your training zone is that you end up with a training zone that might be below your actual capacity, which means that you could potentially miss out on fitness benefits. My suggestion if you are over age 40 is to try it both ways and monitor how you feel during the workouts. Stick with the workout intensity that leaves you feeling at least warm and slightly out of breath but does not put you at risk for injury. You should speak with your doctor if you have any questions about your target heart rate.
NOTE: There will not be any difference in HRmax when you factor in age 40 to the old and new equations. The difference in HRmax between the two equations only becomes significant when you factor in older ages.
Heart rate monitors are devices that measure heart rate in real time. They have grown wildly in popularity over the past 10 years partly due to the miniaturization and accuracy of computer chips. Many athletes use heart rate monitors during their workouts to determine if they are in the proper training zone. But heart rate monitors aren't just for elite athletes. I recommend a heart rate monitor if you like gadgets or think you might like the heart rate data and real-time feedback from your body that these devices provide. The standard design is a strap with a transmitter that you wear around your chest and a wristwatch with a receiver. The chest transmitter detects your heart rate during exercise and wirelessly sends the signal to the wristwatch display for you to see. You can purchase all the bells and whistles with functions like downloading the entire workout to your computer or alarms that let you know when you go too high or too low in your training zone, or you can go minimalist and purchase the basic model that just reads your heart rate. Nike, Polar, and Timex are just three reputable companies that manufacture heart rate monitors.
Heart rate monitoring and training zones aren't for everyone. But don't worry, you can still get a great workout and gain all the benefits of exercise without. All you need to do is listen to your body. You're working out if you feel your heart pumping and you're slightly out of breath. You can also use the Borg Scale of Perceived Exertion (RPE) to measure intensity. Simply select the number from below that best describes your level of exertion.
6 No exertion at all
7.5 Extremely light
9 Very light
13 Somewhat hard
15 Hard (heavy)
17 Very hard
19 Extremely hard
20 Maximal exertion
An exertion level from 13-14 will get you to the middle of your aerobic training zone. For more information about RPE, go to http://www.cdc.gov/nccdphp/dnpa/physical/measuring/perceived_exertion.htm.
Heart rate training zones and heart rate monitoring is:
1. helpful for individuals who want to stay in their aerobic training zone,
2. interesting real-time feedback for individuals who like to know how their body responds to exercise, and
3. important data for athletes who want to get their intervals right.
But even if you never get hooked on the idea of a training zone, you can still get a great workout by listening to your body or using a scale like the RPE. The important point is to get out there and move no matter how you monitor your workout!
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